As a conventional horizontal dry mill, there has been known a type described in JP-U 7-8034B. The horizontal dry mill described in this publication comprises: a circular tubular grinding chamber having a raw material input nozzle disposed at one end thereof, a pulverized material discharge section disposed at the other end thereof, and a separator disposed adjacent to the discharge section; a screw serving as a thrust generating member disposed on the side of the one end of the grinding chamber; an agitation member rotatably provided in the grinding chamber and having a plurality of arms implanted around a rotary shaft to serve as agitation elements; and grinding media filled in the grinding chamber. In this horizontal dry mill, a raw material input into the grinding chamber through the material input nozzle is thrust toward the discharge section by the screw, so that it is agitated together with the grinding media and pulverized by means of mutual friction, shear, etc., whereafter a resulting pulverized product is separated from the grinding media by the separator and discharged from the discharge section.
However, in the above horizontal dry mill, the agitation element is a simple arm. Thus, when a raw material has high flowability, the raw material passes through the grinding chamber without filling an upper region of the grinding chamber and reaches the separator, so that a volumetric capacity of the grinding chamber cannot be sufficiently utilized, and a retention time of the raw material in the grinding chamber becomes shorter, which makes it impossible to effectively utilize the grinding media filled in the grinding chamber, causing significant deterioration in grinding efficiency.
Therefore, the applicant of this application previously offered a horizontal dry mill capable of increasing a retention time of a raw material in a grinding chamber and sufficiently bringing out a grinding ability of grinding media so as to efficiently perform grinding of the raw material, in JP 2007-319726A.
The horizontal dry mill proposed in the JP 2007-319726A comprises: a tubular grinding tank having a grinding chamber; a raw material input section provided at one end of the grinding tank; a media separation section provided at the other end of the grinding tank; grinding media set in the grinding chamber; and an agitation member provided between one end and the other end of the grinding chamber and disposed rotatably about a horizontal rotation axis, and is characterized in that the agitation member comprises a plurality of circular truncated cone-shaped discs each having a vertical surface and a taper surface with a size which gradually decreased toward the one end of the grinding tank, and a plurality of pins, which are arranged alternately, wherein each of the discs is formed with a communication hole extending obliquely in a thickness direction of the disc, and wherein the communication hole has a first opening located on the vertical surface at a position on a relatively inner peripheral side of the disc, and a second opening located on the taper surface at a position on an outer peripheral side of the disc with respect to the first opening.
As described above, in the horizontal dry mill proposed in the JP 2007-319726A, the plurality of circular truncated cone-shaped discs are used as a part of agitation elements of the agitation member, and the communication hole is formed in each of the discs to extend obliquely in the thickness direction of the disc, specifically, the communication hole is configured to have a first opening located on the vertical surface at a position on a relatively inner peripheral side of the disc, and a second opening located on the taper surface at a position on an outer peripheral side of the disc with respect to the first opening, so that the raw material and the grinding media on a downstream side of the disc partly flow back to an upstream side of the disc through the communication hole. The backflow makes it possible to increase a retention time of a material to be ground in the grinding chamber, and perform sufficient grinding. In particular, the backflow makes it possible to give large kinetic energy to the grinding media and increase a probability of contact between the grinding media, thereby enhancing grinding capacity. These allow the horizontal dry mill to perform fine grinding of the material.
Meanwhile, in the horizontal dry mill having the above structure, as for supply of a raw material to the grinding chamber, a raw material to be pulverized is discharged from a constant feeder, and directly input into the grinding chamber by means of gravity fall. In the horizontal dry mill, a raw material having a particle size of several 100 μm can be pulverized into several μm in just one pass. However, when the particle size is reduced to be equal to or less than 10 μm, cohesion rapidly becomes stronger, so that such particles are liable to re-cohere. Although a grinding aid is added to address prevention of such cohesion, the media and pulverized particles are pushed up to the material input section by a centrifugal force produced by rotation of the agitation member, and the material input section is apt to be clogged due to adhesion and buildup of particles increased in cohesion force, thereby precluding operation.
Moreover, a raw material with a low specific gravity has a disadvantage, such as a situation where it is blown up by a centrifugal force, thereby becoming incapable of being input into the grinding chamber.
In the horizontal dry mill proposed in the JP 2007-319726, the above disadvantage has been improved to some extent by providing a raw material supply section having a given length, on the side of one end of the agitation member, to allow a raw material to be input thereinto, and supplying the raw material input in the raw material supply section, to the grinding chamber by a screw or the like, instead of directly inputting the raw material into the grinding chamber in which the agitation member exists.
However, in this material supply section, a screw or the like is also used, so that, due to a centrifugal force produced by rotation of the screw or the like, blow-up and adhesion of the raw material, etc., to the raw material supply section, is likely to occur.